The present invention relates to processes for making products and to the products made by the processes. More particularly, it relates to processes and/or methods and apparatus for forming and machining a piece of stock or a work piece to produce a monolithic product, particularly a product comprising integral skin and frame or skeleton portions, and more particularly, a product suitable for use in the aircraft industry and comprising integral skin and frame portions.
Currently, the majority of structural members, particularly aircraft skin and airframe structural members, are built up from sheet metal using a frame structure (e.g., stringers and ribs), then attaching a skin to the frame structure using mechanical fasteners such as rivets or the like. In the aircraft industry, structures made in this manner may be referred to as “stick built” or “built up” structures.
In the aircraft and other industries, there are parts and components, including plates, skins, hull and/or shell members, machined whole from plate material, castings and forgings, and extrusions. Such parts and components may comprise non-metallic composite parts, but, regardless of the material, historically and generally they are connected to frame structures using mechanical fasteners. Particularly referring to aircraft and/or airframe structures, such as wing skins, such structures would generally be machined, formed and then assembled. The assembly generally consists of stringers and/or ribs being mechanically fastened to a wing skin material after forming the wing skin. The purpose of the stringers and ribs is to support and stiffen the finished structure.
The use of monolithic structures, including aircraft airframe structures, as a replacement for “stick built” structures is known. Generally, a common approach has been to machine a structure or component, for example a skin plate, complete from a selected piece of raw material. This process has also included finish machining and then final forming to achieve the desired shape, including “shot-peen forming” and/or “plier forming.” One problem with this approach is that there may be significant residual stress in the product, and this may lead to increasing the cross-section of frame members or the skin itself to meet required tolerances and safety requirements.
It would be advantageous if a monolithic wing skin could be created to incorporate, by machining, the stringers and ribs into the finished product thereby eliminating a number of fasteners, and the labor required by the fastening process. It would be desirable if such a monolithic wing skin could be machined from a single piece of material thick enough to produce the desired shape without any forming of the material. Historically, however, a limitation to this approach has to do with the raw material properties of thick plate. For example, a thick aluminum plate does not have the same physical characteristics throughout its cross section. As a result, material limitations and strength requirements drove designs to thicker and heavier cross sections, particularly of the ribs and stringers, thereby introducing a weight penalty associated with known methods of making monolithic parts.
U.S. Pat. No. 4,941,338 is directed to a device for cold forming of ferrous and non-ferrous metal sections. In the process, pre-tensioning is applied to an initially essentially straight metal section, which is clamped at both ends. The tensioned metal section is bent in a rotating bending tool, and in order to achieve three-dimensional forming of the metal section, the bending tool effects a controlled movement in three spatial axes, whereby the metal section is pressed with a backlash-free guide system, at least in the forming region against the bending tool and guided by positive engagement.
U.S. Pat. No. 3,861,009 is directed to a process and apparatus for forming structural members wherein a continuous cold forming process is provided for making structural members from a strip of expandable metal.
U.S. Pat. No. 6,071,360 is directed to controlled strain rate forming of thick titanium plate. Although it involves forming a plate, then completing a part by machining the formed plate, the forming takes place at an elevated temperature.